Nest 2nd Gen Installation

I'm installing a Nest Gen 2 Thermostat in my Eichler and am using a Honeywell Relay RA89A to convert from high voltage to low voltage.
I'm wondering if I can place the relay right behind the thermostat and if this would cause any issues.

The other ends of the 120V wires behind your original thermostat are in the small electrical box on the garage wall that connects power to your recirculating pump. This location is always the simplest for the relay. Nice that you posted the inquiry before taking your thermostat wall apart. When I installed the relay, more than 20 years ago, I attached it to the metal lid of the box. Your Honeywell assembly has a box that you could install right over the existing box....making for a very neat install.

It sounds like Randy answered your question. But I just wanted to say that I'm very interested in your Nest project and how it works with radiant heat. I'm specifically interested in if the Nest can manage the huge lag in time to heat the space to the desired temp--that's always interested/confounded me. I can see how the features like determining if someone is home and adjusting accordingly would be nice with forced air where you can increase room temperature in 10 minutes, but I'm curious how it would work with radiant, or if features like that would just be disabled.

We installed a Nest at Christmas, and I managed to grab the raw data for house temperature, thermostat setting, and boiler status for the last week. (Ask for details; it involves writing a computer program to poll the Nest servers every 10-20 minutes to get the latest data.)

I'd adjusted the Nest so it would set the temperature at 68 at 6 am; for the last week, the Nest has been turning on the boiler at 4am so the temperature will be up by 6am, and turning off the boiler while the temperature is still below the thermostat setting. It'll be interesting to see how it does when the temperature dips again, but so far I'm optimistic.

Our thermostat has auto-off set, but the house hasn't been empty so I haven't seen what the thermostat will do.

Thank you everyone for the responses.
I have all the equipment I need before I begin -just one last question.
I opened up the old thermostat and there are two wires there - a black one and a white one.
I'm not sure which wire connects to which spot on the nest.

(bump) I have the same question as Supersonic Buddha. Once you make the line- to low voltage conversion at the pump/boiler side of the wiring, you have not altered the actual fat black and white line voltage wires that were connected to the line voltage thermostat the Nest is now replacing. Do you use wire screws to get to a thinner gauge or merely use the line voltage black and white? For someone who has done this, a picture of the back of the connected nest with the black and white line voltage wires installed\spliced, etc. would be the last piece in finishing this project before next winter.

ANSWERS TO THE WIRING ISSUE FROM NEST. I was able to discuss the Eichler radiant floor boiler/line voltage wiring situation with the Nest Support Team today. While they cannot officially recommend the step down solution, the feedback from customers and installers is that it does generally work, but there are some considerations. They do NOT recommend using the subset of Nest-certified installers who are home automation companies, but rather those that are true HVAC companies. The install process is as : install the line to 24V step down at the furnace side of the equation (the Honeywell Relay RA89A works nicely). Then, at the thermostat side, first, the line voltage thermostat is removed. Then the 2- 10 gauge (line voltage) wires from the original thermostat are connected to 2- 18 gauge (thinner)pigtail wires with wire nuts. Now they will fit into the back of the Nest. Based on the function of each of the original line voltage wires, the two (now smaller gauge) wires will be connected to the proper terminals of the Nest. Installation complete*. Now the fine print (warning) from Nest and rationale for using an HVAC guy and not a home automation installer- Because of the step down relay, and, equally important, transition from the 10 gauge wire down to an 18 gauge wire, the electrical resistance on the system will increase. Whether this increased resistance will affect the Nest's electronics, or the electronics on the boiler-side, is not consistent from install to install. Having the HVAC installer measure the increase in resistance to determine whether it is material is critical for safety and not frying the electronics (as Nest will not honor the warranty if it is determined to have been a resistance issue that caused it to fail). Nest's own official, unofficial guidance, is that the increased resistance shouldn't be problematic, but COULD be.

One last note on this project which I failed to mention and is the same reason many people think the relay is broken when they install it. This will NOT work if your house is wired with the power between your thermostat and your pump/boiler going through the breaker box to make that connection! Thankfully, most homes have the power to the pump going into the utility closet from the breaker panel and then the line voltage wires to the thermostat running through the walls from the pump do not go back to the breaker box, they go straight to the thermostat.